1. Field of the Invention
This invention relates to electrical engineering and more particularly, to excitation systems for synchronous machines. By the excitation system described herein is meant a set of devices intended to supply a field winding with an automatically controlled direct current, to protect the field winding and an exciter against overvoltage during transients in a synchronous machine, and to perform some other functions.
2. Description of the Prior Art
Hitherto, an excitation system for a synchronous machine has been made up of an exciter having its output coupled to a starting-protecting unit comprising a resistor connected in series with a thyristor switch controlled by a voltage-sensitive threshold element that can be made to operate at a certain voltage setting (Vershinin P. P., Hashper L. Ya. "Synchronous Machines in Metal-Making", Moscow, 1974).
This excitation system is insufficiently reliable because any failure in the starting-protecting unit (an open circuit, loss of control, or short-circuited thyristor switch) entails the risk of an emergency shut-down of or damage to the exciter and the synchronous machine.
Another approach has been an excitation system for a synchronous machine, comprising an exciter with its output connected to a starting-protecting unit containing parallel branches, each consisting of a resistor connected in series with a thyristor switch in which a voltage-sensitive threshold element is connected between the anode and the gate ("Catalogue of Series KTU Complete Thyristor Devices" OKP. 341.631, Tallin, 1978).
This excitation system is more reliable because an open circuit in one of the parallel branches, provided the other branches operate normally, will not cause the excitation system to lose its protection functions, and, as a consequence, no emergency shut-down of the exciter and the synchronous machine will be required.
The presence of the parallel branches, however, makes less reliable the starting of the synchronous machine and some other modes of operation accompanied by the flow of considerable currents through the starting-protecting unit, becuase the threshold elements controlling the thyristors of the thyristor switches differ in voltage setting and thyristor turn-on time. Just as the first parallel branch of the starting-protecting unit is energized (for example, in starting the synchronous machine), the voltage across the field winding of the synchronous machine drops so that the remaining parallel branches of the starting-protecting unit can be energized with difficutly or, sometimes, not at all. As a result, one parallel branch takes all of the starting current, is overheated, and impairs the reliability of the entire excitation system.